As a sensor for detecting an amount of particulate matter (hereinafter, also referred to as PM) contained in exhaust gas, there has been known a PM sensor of electrode type which has a pair of electrodes disposed in opposition with each other through an electrically insulating layer and which makes use of an electrical characteristic in which the value of a current flowing between the electrodes changes according to an amount of PM deposited between the electrodes.
As a method of diagnosing the failure (abnormality) of the particulate filter by using the electrode-type PM sensor, there has been known a method in which a comparison is made between an output value of the PM sensor (an electrical signal correlated with the value of the current flowing between the electrodes of the PM sensor) at the time when a predetermined period of time has passed from a point in time at which processing for removing the PM deposited between the electrodes of the PM sensor (hereinafter, referred to as “sensor regeneration processing”) has ended and a predetermined threshold value, so that when the output value of the PM sensor is larger than the predetermined threshold value, it is diagnosed that the particulate filter is in failure (see, for example, a first patent literature).